The range finder has been diversified in these days, where the non-contact measuring method such as the light propagation time measuring method and binocular stereopsis method, etc., has been employed more frequently than the contact measuring method. Specially in terms of the accuracy, the light-section method has been prevailing in the application fields such as the robot handling, etc.
Hereafter, a construction of a general measuring equipment using the light-section method will be explained. FIG. 4 illustrates a construction of a conventional range finder using the light-section method. A laser beam projector 301 emits a laser beam on an object 302 to be measured. When a CCD camera 303 takes an image of the object 302, a position of the laser beam spot projected on the CCD camera 303 varies depending on a position of the object 302 (triangulation method). In order to measure the position of the laser beam spot, an analog video signal on the CCD camera 303 is converted into a digital signal by an A/D converter 304. The digital signal is converted into a binary image data by a binarizing circuit 306 on the basis of a predetermined threshold 305. A labeling unit 307 applies the labeling processing to this binary image data, and a centroid detection unit 308 detects a centroid of the image data, whereby the position of the laser beam spot on the image can be detected. If the position of the laser beam spot on the image is detected, a distance calculation unit 310 is able to calculate the distance by using optical parameters (focal length, distance between the laser projector 301 and the CCD camera 303, and angle formed by the two) or a calibration table 309.
Here, in the conventional construction, it is a premise that the image of the laser beam spot projected on the CCD camera 303 is brighter than those of the other parts on the image; however in the outdoors, in a place where the brightness is influenced by the sunbeam, the output power of the laser is inevitably raised, which leaves a problem in terms of safety. Further, since the intensity of illumination complicatedly varies in the outdoors, the threshold cannot be predetermined.
Accordingly, the inventor proposed an equipment in which the coordinate transformation processing is applied concurrently to the image information obtained from a CCD camera, and thereafter the superposition processing is applied to the image with the coordinate transformed, whereby the surrounding light is removed and only the laser beam is identified (see JP-A 8-94322).
However, still in this proposal, when a moving object comes in, or when a background object swings owing to the wind or the like, the images influenced by the moving objects cannot be removed, and are left as noises.